How to deal with additive noise?

Discussion in 'Wireless & RF Design' started by davidGG, Sep 8, 2013.

  1. davidGG

    Thread Starter Member

    Dec 22, 2012
    51
    0
    Hello everybody,
    I'm using an RF module to send an 8 bit binary number through a microcontroller. The operation is simple; when I press a button on the transmitter end, the μC sends the information and when received by the other end, the receiver μC toggles an LED.
    My problem is that there is a lot of noise. I have two receivers (433MHz), one is pretty good one which works for about every 8/10 button clicks, and the other is really bad which works for about 1-2/10 button clicks. I'm using a linear wire antenna.

    Here is an oscilloscope image of both receivers (this noise is when receivers are idle):
    Good receiver:
    [​IMG]

    Bad receiver:
    [​IMG]
    exact product I am using

    Here is my μC code (ATMEGA 32A):
    Transmitter: http://pastebin.com/dX72mAkJ
    Receiver: http://pastebin.com/BTDhdQ3u

    Both of my receivers and transmitter are OOK/ASK modulation, is this the problem? I've tried adding capacitors across data/VCC and data/gnd pins to get rid of the noise but I have had no luck.


    By the way, my 'good receiver' is damaged. When I first got it, I accidentally connected it to a 9V battery and it started smoking. It still works, but it gets VERY HOT, but it doesn't smoke. Is it still ok to use?

    I appreciate the help,
    Thank You.
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    davidGG likes this.
  3. davidGG

    Thread Starter Member

    Dec 22, 2012
    51
    0
    Thank you so much. Now when using my 'good receiver' data transmission is flawless. Had no idea that baud rate makes so much of a difference. Thank you.

    Just one more thing, here is the oscilloscope image of the receiver after I made adjustments from your website.
    [​IMG]

    why does my receiver still look like that? the left portion is when data is being received and the right portion is idle. Any ideas on how I can get it to look like your scope view?
    Maybe it is because my good receiver is sort of broken due its really high temperature? Even then, my bad receiver which I believe is not damaged has a lot of static and works even worse.

    OR

    can it be the oscilloscope I am using? It is a USB/PC scope that I got from ebay for about $30 (low budget). However, all other signals seem pretty accurate.
    Thank you.
     
    Last edited: Sep 9, 2013
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    It's normal to get some noise out even when there is no local signal, because the receiver will crank up it's gain until it finds something even when YOU are not transmitting. :)

    You said your receiver "gets VERY hot"! That is not a good sign. I suggest you buy a new one and at least clear up whether yours is damaged or not.

    Re the scope traces, it is well worth the effort to do a test like I did, where you transmit a simple 1kHz squarewave (logic HI for 500uS, logic LO for 500uS).

    That is very easy to see on the scope, and you can then see if your local 1kHz squarewave is strong enough to totally swamp remote signals when you are transmitting. That is about the best you can hope for.

    When you are not transmitting, you WILL get noise coming out of the receiver, so you need a way to check if that signal is a real signal or noise. That is done by encoding, ie; send a preamble and only if the preamble meets certain conditions you then allow data bytes after it.
     
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